The invention relates to a communication system based on an integrated services digital network ISDN, comprising a private branch exchange coupled to a public network, the private branch exchange being coupled to a passive bus for connecting ISDN supported terminal stations which communicate with the public network via the private branch exchange, signalling being effected over an ISDN D-channel.
The invention likewise relates to a private branch exchange for a communication system based on an integrated services digital network ISDN, the private branch exchange being coupled to a passive bus for connecting ISDN supported terminal stations which can communicate with a public network via the private branch exchange, signalling being effected over an ISDN D-channel.
A communication system of this type is known from the handbook entitled "Datenkommunikation", D. Conrads, Friedr. Vieweg & Sohn, 1990, pp. 199-219. On page 200 of said handbook a system architecture of an ISDN communication system is shown with an ISDN (Integrated Services Digital Network) as a public network, in which communication from one terminal station to the other is effected via local exchanges, and on page 201 there is described that communication between local exchanges and terminal station may be effected via so-termed 2B+D Basic Rate Access, without framing that is, over 2.times.64 kbit/sec B-channels and a 16 kbit/sec D-channel, or, as described on page 205, if a private branch exchange is linked to the network via so-termed Primary Rate Access, in Europe 30B+D, or at 2.048 Mbit/sec. In the latter case Basic Rate Access may occur between the private branch exchange and terminal stations coupled to the private branch exchange. In such an ISDN, data or speech communication may be effected via ISDN B-channels through so-termed circuit or packet switched connections between subscribers, whereas the D-channel is used, for example, for signalling purposes. In addition to being connected to an ISDN, ISDN terminal stations can also be connected via network interfaces to a conventional public telephone network, a so-termed PSTN (Public Switched Telephone Network), via a private branch exchange. On page 207 of the handbook, ISDN reference points are indicated for an ISDN private branch exchange connection, the so-termed S.sub.0 interface being intended for connecting ISDN supported terminal stations thereto. Customary analog terminal stations are coupled to the S.sub.0 interface via what is commonly referred to as a Terminal Adapter. The S.sub.0 interface may be coupled to a so-termed U interface or U reference point via a Network Termination, from which reference point a connection to the PABX (Private Automatic Branch Exchange) or ISPBX (Integrated Services Private Branch Exchange) may be realised via an existing two-wire line. In the case of an ISPBX, ISDN supported terminal stations may also communicate with the ISPBX via an S.sub.0 interface by means of a four-wire line. This is shown in FIG. 98, on page 207, of said handbook for connecting, for example, a passive bus to the ISPBX. In the case of a PABX, such a passive bus is connected to the PABX by a U interface and a two-wire line. Various ISDN supported terminal stations, a maximum of 8 according to CCITT Recommendations, two terminal stations of which can be active simultaneously via the B-channels, can be connected to the passive bus as this is described on page 204. Page 204 further states that only ISDN supported terminal stations can be connected directly to the S.sub.0 bus, and that conventional terminal stations such as X.21 or X.25 stations and the like, are to be connected to the S.sub.0 bus via a Terminal Adapter in which digitization and signal adjustment is effected. Such Terminal Adapters are often complicated and relatively expensive. Furthermore, an article in Philips Telecommunication and Data Systems Review, Vol. 48, No. 1, March 1990, pp. 20-27, "Facilities for users of SOPHO-SET feature phones", has disclosed telephone sets which exhibit a great deal of additional features compared to simple telephone sets. These telephone sets, having a display, LED indicators and various function keys, communicate with a digital PABX or private branch exchange over a separate signalling path which enables exchange of messages with the PABX without a telephone call being made. In digital SOPHO-S telephone sets a D-channel in a separate 2B+D two-wire line in the firm-owned set is used for such traffic of messages. A great number of features exhibited by such a telephone set in combination with a private branch exchange, such as featuring informative messages on the display, informative signalling by means of the LED indicators, executive/secretary features such as absence signalling etc. are not supported in ISDN supported terminal stations.